Dry-process developing devices using a powdery developing agent have widely been employed in image forming apparatuses such as electronic copiers, printers and facsimiles, in which a latent electrostatic image formed on a latent image bearing member is visualized with a developer to obtain a recorded image.
In recent years, color image forming apparatuses using electrophotographic process have broadly been employed, and digitized images are easily available. Thus, it is required to make an image to be printed at higher definition. While studying higher resolution and gradation of an image, as an improvement of a toner which visualizes a latent image, it has been studied to further conglobate and minimize in particle size for forming the image at high definition. And, since in the toners produced by the pulverizing methods, their conglobation and minimization are limited, so-called polymerized toners produced by a suspension polymerization method, an emulsification polymerization method and a dispersion polymerization method capable of conglobtaining and minimizing in particle size have been being employed.
In the production method of polymerized toners, toner materials having relatively low resistance are localized in the vicinity of the surfaces of toner core particles. Thus, the formed polymerized toners have low chargeability to cause background smear. In addition, the polymerized toner has a small particle diameter and thus has increased adhesive force to members, thereby raising problems such as filming and a drop in transfer efficiency. Furthermore, the polymerized toner is highly spherical to cause cleaning failure.
In view of this, attempts have been made to modify the surfaces of toner core particles to solve the aforementioned problems. The method for surface modification is, for example, dry methods in which fine particles are made to adhere onto the toner surfaces by the action of mechanical impact, and wet methods in which a resin dispersing agent is added to a dispersion liquid containing toner particles dispersed in a solvent, wherein the resin of the resin dispersing agent is different from the resin forming the toner particles.
Regarding the dry methods, disclosed is a toner including toner base particles and fine particles embedded in the surfaces thereof, wherein the toner is produced by adding the fine particles to the toner base particles heated to a temperature near their softening point, followed by stirring and mixing (see PTL 1). Also, disclosed is a toner including fine resin particles and toner core particles which are covered with the fine resin particles by the action of mechanical impact (see PTL 2).
However, in these dry methods, the fine particles cannot be uniformly and sufficiently attached or adhered to the toner base particles and toner core particles. As a result, the fine particles are exfoliated from the toner base particles and toner core particles to cause problems such as filming and adhesion.
Regarding the wet methods, disclosed is a method in which the surfaces of toner core particles formed of first resin particles and a colorant are partially or totally covered with second resin particles (see PTL 3). However, according to this method, the toner core particles are covered with the second resin particles so sparsely and ununiformly that background smear and toner storage stability cannot be sufficiently improved, although cleanability is improved. In addition, degradation of transferability occurs.
Also, disclosed is a toner including toner core particles and convex portions with an average diameter of 100 nm to 500 nm which are provided on the surfaces of the toner core particles, wherein the toner core particles are covered with the convex portions at a coverage rate of 10% to 80% (see PTL 4). However, according to the production method described in Examples, the protrusions of the toner are not uniform in size, and thus the toner cannot solve problems such as background smear. The binder resin forming the convex portions has high polarity to greatly change depending on the environment and thus, is insufficient in improvement of heat resistance storage stability.
Also, disclosed is a method in which fine resin particles are added in advance to an aqueous phase for fusion to control the particle diameter (see PTL 5). However, in this method, the fine resin particles are incorporated into toner core particles, and as a result, the toner core particles cannot be covered with the fine resin particles in such an amount that heat resistance storage stability is improved.
Also, disclosed is a toner having a core-shell structure (see PTL 6), but in this toner, cores are totally covered with shell layers, leading to considerable degradation of fixing property.
In addition to the above-described surface modification, some attempts to solve these problems by appropriately selecting external additives have been made. In particular, there have been various proposals utilizing hydrophobicity and low surface energy of silicone oil.
For example, it is disclosed that qualities of both transfer and fixing are kept in a favorable balance by defining the silicone oil release rate of fine inorganic particles each containing silicone oil (see PTL 7). Also, there is disclosed fine silica particles treated with silicone oil and having two peaks in the particle size distribution thereof (see PTL 8). Also, it is disclosed to use as external additives aggregates of fine particles treated with silicone oil and fine inorganic particles (see PTL 9). Also, it is disclosed to use as external additives fine inorganic particles treated with silicone oil and fine inorganic particles treated with a silane coupling agent (see PTL 10). However, any of these methods is not sufficient to retain transferability and abrasion resistance for a long period of time in a wide range. Attachment of an excessive amount of an external additive would degrade fixability and also raise contamination of a released external additive.